Question: how exactly does migration work? One would assume that people moving away from one planet go to another, and the other way round, that people coming to a planet have left somewhere. But from what I read here (migration out of thin air), it sounds to me like this is not the case, that the Nomadic-trait is essentially working like the Fertile-Trait. I am confused.
Its pretty simple actually, Migration is a function of the relationship of emigration push to immigration pull for a group of related planets.
1. A planet with more emigration push than immigration pull will have net Emigration. This will "push" a percentage of that planet's growth to the immigration pool of all related planets. That percentage caps at 95% of a planet's final growth being converted into emigration, at a
net emigration push value of 100.
2. Planets with net immigration will have a portion of all the growth being added to their related immigration pool added onto their final, in an amount relative to the immigration pulls of related planets. IE, if Earth is pushing 5 growth into its related pool, and Mars has a net immigration of even 1, it will get all 5 pops worth of growth added on. However, if both Mars and Saturn each have a net immigration of 1, Earth's 5 emigration will get split between Mars and Saturn, so they'll each get 2.5 added growth.
2.1 Immigration Pull cannot steal growth from planets that do not already have net emigration themselves. If all planets in a related group have net emigration, no one will migrate. If all planets in a related group have net immigration, no one will migrate. Immigration pull only serves as a weight to determine which planets with net immigration will receive any pre-existing net emigration. The following is an example screenshot from my current testing game:
https://imgur.com/DYIeCNu
In it I have every planet except the colony shown at net emigration. The colony shown has a net immigration of only 7, yet its receiving ALL emigration being pushed from all related planets, because its the only available target.
2.2 Total base immigration is usually capped at 5 per planet. Meaning that even if Earth is emigrating 10 growth per month, if only Mars is available to receive that emigration, 5 growth worth of migration is being lost per month. This growth is magically dying in transit.
3. Nomadic multiplies the final added growth a planet is getting via migration by 115%, so indeed this growth is magically coming out of thin air. Earth is getting 95% of its growth value subtracted, and Mars is getting 109.25% of Earth's growth value added,
if the pop in question thats
growing has the nomadic trait and exists in Earth's emigration pool.
4. Note that while the species distribution of a planet with net Emigration Push
does weight the growth species of planets receiving that immigration, that emigration weight can be severely outvalued by mechanics like Habitability, or new_pop_species_div, meaning that planets can receive tons of growth from immigration, but produce species completely disproportionate to the species being emigrated. These pops can essentially magically change race in transit.